This invention generally relates to lasers and more particularly this invention relates to optical heterodyne frequency generation.
There are many test applications that require an optical signal to be modulated at a radio or microwave frequency. Such applications include receiver testing, optical-based microwave generation, testing of microwave components, direct optical injection of source signals for high frequency testing (photo-detector on chip) and fiber delivery of microwave signals for remote antennae testing. Presently there are three methods for high frequency optical modulation. The first technique, known as an impulse technique, involves applying an impulse from a mode locked laser. Using a spectrum analyzer, a frequency domain response can be obtained. For modulation frequencies in the hundreds of gigahertz (GHz) this can be expensive. Furthermore this method is difficult to calibrate since the frequency amplitudes of the mode-locked pulses are generally not known. Recovering accurate phase information is even more difficult and often impossible.
An alternative is to modulate a laser using an RF or microwave modulator. However, for modulation at high frequencies it is desirable to use a frequency synthesizer that is both NIST traceable (i.e., calibrated in a manner traceable to a National Institute of Standards and Technology reference source) and stable with respect to both frequency and amplitude. Such synthesizers can be expensive for frequencies above a few tens of gigahertz. Furthermore, although the modulation frequency can be calibrated using this technique, it is difficult to calibrate the amplitude of the modulated signal.
Thus, there is a need in the art for a high-speed verifiable frequency calibration technique in optical heterodyne systems that avoids the costs associated with high-speed frequency detector electronics, and can extend to frequencies where it is currently not possible.
The disadvantages associated with the prior art are over come by embodiments of the present invention directed to methods and apparatus for method for calibrating a frequency difference between two or more lasers over an extended frequency range. Additional embodiments are directed to optical signal generators that employ such a method or apparatus.
According to one embodiment among others the method may generally proceed by tuning the lasers in coordination with respect to one or more readily characterized narrow frequency ranges to characterize one or more tuning parameters of each of the lasers over the extended frequency range. By way of example, the frequency difference may be calibrated with respect to the tuning parameters over first and second narrow frequency ranges and the resulting frequency difference calibrations may be coordinated with respect to the tuning parameters over the extended frequency range.
Embodiments of the method for calibrating the frequency difference between two lasers may be embodied in a computer readable medium having embodied therein a set of computer readable instructions that implement the method described above.
The sequence of events set forth in the method may be carried out by a suitably configured apparatus. According to one embodiment of the invention among others, such an apparatus may include first and second tuning controllers respectively coupled to the first and second lasers, an optical coupler optically coupled to the first laser and the second laser, a frequency detector coupled to the optical coupler and a controller coupled to the frequency detector and the tuning controllers. The controller may include a processor and a memory containing processor executable instructions for calibrating the two lasers in accordance with the method described above. Such frequency calibration apparatus may be incorporated with the first and second lasers into a signal generator apparatus. The frequency detector may include a local detector optically coupled to the optical coupler, a phase locked loop coupled to the local detector and the controller, an integrator coupled to the phase locked loop and the controller, a direct digital synthesizer coupled to the phase locked loop and the controller, and an oscillator coupled to the direct digital synthesizer.
Embodiments of the present invention provide for frequency calibration between two or more lasers over a frequency range greater than that of the frequency detector used and/or its associated electronics. This enables frequency calibration over a broad range while using a relatively inexpensive low-frequency reference.